Plant Molecular Biology 38: 279–310, 1998.
© 1998 Kluwer Academic Publishers. Printed in The Netherlands.
Intercellular protein trafﬁcking through plasmodesmata
Department of Botany, Oklahoma State University, Stillwater, OK 74078, USA (e-mail: email@example.com.-
Key words: plasmodesmata, protein trafﬁcking, viral movement protein, virus movement, plant development, plant
During plant morphogenesis, groups of cells differentiate to form specialized tissues possessing distinct structures
and functions. Cell specialization is a result of speciﬁc gene expression at the individual cell level. Coordination of
differential gene expression among cells requires that cells communicate with one another. Plasmodesmata provide
a cytoplasmic pathway for direct intercellular communication. Recent discoveries that macromolecules such as
transcription factors, viral proteins, and plant defense-related proteins can trafﬁc through plasmodesmata suggest
that intercellular protein trafﬁcking is potentially an important means to regulate plant developmental processes,
physiological functions, plant-pathogen interactions, and plant defense reactions. Thus, elucidating the speciﬁc
functions and mechanisms of intercellular protein trafﬁcking has broad implications in understanding how a plant
develops and functions at the molecular level. This review is to provide an update on this rapidly developing area of
plant biology, with emphasis on the discussion of possible mechanisms underlying intercellular protein trafﬁcking.
The formation of a multicellular organism results from
cell division, differentiation and growth. Cell division
in plants is incomplete in that the daughter cells remain
connected cytoplasmically via plasmodesmata (PD)
that are formed within the developing cell walls by
entrapment of the endoplasmic reticulum (ER) [108,
148]. The developmental fate of a cell is rarely de-
termined by its lineage alone. Rather, the physical
position of the cell in a plant plays a more decisive
role in shaping the developmental course of the cell
[244, 261]. A cell clearly senses its physical position
by interacting with its neighbors.
Speciﬁc gene expression at the individual cell level
establishes the identity and function of a cell. This
level of gene expression can be regulated by trafﬁck-
ing of informational molecules between the cytoplasm
and nucleus through the nuclear pore complex (NPC).
Coordination of gene expression between cells can be
achieved by PD-mediated cell-to-cell communication.
Figure 1 highlights the concept that coordinated NPC
and PD trafﬁcking of informational molecules is fun-
damental for the regulation of differential gene expres-
sion and morphogenesis in plants. Nuclear transport
is reviewed by Heese-Peck and Raikhel in this issue
. It should be noted that cell walls also play an
important role in cell-to-cell signalling (e.g. ).
PD can regulate cell-to-cell communication by
more than one means. Developmental changes in PD
structure, frequency, and size exclusion limit (SEL)
can lead to the establishment of the so-called symplas-
mic domains. Within each domain, the metabolism
and function of all cells are presumably synchronized.
This subject has been discussed extensively in sev-
eral reviews [68, 148, 157, 201]. Given the theme of
this special issue of the journal, the present review
will focus on intercellular trafﬁcking of proteins, a
PD function only recognized recently from a number
of experimental systems. Examples of this function
include cell-to-cell trafﬁcking of viral proteins, plant
defense-related proteins, plant transcription factors,
and other plant proteins whose functions remain to
be elucidated. An important concept developed from